1. Field of the Invention
The present invention generally relates to digital to analog conversion in a communications system transmitter.
2. Background Art
An increasingly popular standard in conventional communication systems is the Data Over Cable Service Interface Specification (DOCSIS). Communications systems that comply with these standards, particularly DOCSIS upstream standards, typically include transmitters that have a single digital to analog converter (DAC). Each of these transmitters, mainly used in upstream communication channels, typically includes a low-pass filter coupled to an output of its DAC and a power amplifier connected to an output of the anti-aliasing filter.
The anti-aliasing filter is provided to minimize aliasing components in the output signals. The power amplifier is provided to achieve the DOCSIS recommended output power level of about 58 dBmV (decibel millivolts).
An additional (Euro) DOCSIS requirement is the ability to achieve a DAC output signal frequency of roughly 65 megahertz (MHz). That is, in conventional upstream (Euro) DOCSIS based transmitters, a single DAC must be able to run at a sampling rate of at least 130 MHz according to general sampling theory (Nyquist Theorem).
Significant challenges exist in using anti-aliasing filters and power amplifiers in the manner noted above. The greatest of these challenges is that the power amplifiers typically used are relatively large in size. At low DAC sampling rates, e.g. 130 MHz, an expensive anti-aliasing filter is required and this filter may also attenuate the desired output signal, therefore requiring even higher output power from the power amplifier to compensate for these losses. The large size power amplifier is responsible for the consumption of significant amounts of integrated circuit (IC) real estate, or one may need to use an external power amplifier to generate the required high output power.
What is needed therefore is a method and system of converting digital signals to analog domain in a manner that minimizes aliasing components in the output signal. What is also needed is a method and system that can also achieve or exceed the output power requirements of standards, such as DOCSIS.